1. Field of the Invention
The present invention relates to an image reading apparatus which reads an image on a document by converting reflection light rays from an image bearing surface of the document to an electrical signal, and an image forming machine provided with the image forming apparatus.
2. Description of the Related Art
Conventionally, there has been known an image forming machine, such as a copying machine and the like, which is provided with an image reading apparatus or image reader for reading an image on a document by converting reflection light rays from an image bearing surface of the document to an electric signal.
An image reader provided in a copying machine includes a light source for irradiating light rays to a contact glass on which a document is placed, a housing formed with a slit for passing reflection light rays from the contact glass which is irradiated by the light source, a photoelectric conversion element for converting an optical image of the light rays having passed through the slit of the housing to an electric signal. The image reader forms a copy image (data of the read document image) by passing reflection light rays from the contact glass through the slit of the housing, and guiding them to the photoelectric conversion element.
Recently, the size of an image reader is required to be reduced in view of the demand of reducing the installation space for a copying machine in an office and the like. The reduction of the longitudinal size of the housing for the demand requires the size of the slit to have a smaller allowance for reflection light rays from an end edge of the document placed on the contact glass.
In the conventional image reader as mentioned above, reflection light rays L from an area outside an image reading area of a contact glass 8 is reflected on a region in the vicinity of an end portion 275b of a housing 275 which defines an end of a slit 275a, and further reflected on the contact glass 8, finally enters the slit 275a, as shown in FIG. 9. In other words, not only reflection light rays from the image reading area, but also reflection light rays L from the area outside of the image reading area of the contact glass 8 are permitted to come to the photoelectric conversion element. For this reason, it is hard to convert only document information of the image reading area to the electric signal properly. For example, there has been a problem that a flare (a blur of an image) occurs in the vicinity of an end edge of the copy image since a concentration difference becomes greater between the end edge of the copy image and a central portion thereof as approaching the end edge from the central portion, as shown in a concentration graph of FIG. 10.
In order to solve the problem, peripheral members, such as the housing and the like, are coated with black paint to absorb reflection light rays from the area outside the image reading area, and suppress reflection light rays from the area outside the image reading area from entering the slit of the housing, and thus suppress the occurrence of a flare on a copy image (see Japanese Unexamined Patent Publication No. Hei 6-90328, for example.)
The publication of Hei 6-90328 discloses an image reading apparatus in which reflection light rays from a document irradiated by a light source is passed through a slit defined by two plate members arranged opposite to each other at a predetermined space and focused onto a reading sensor by means of a focusing lens. Further, the respective outer surfaces of the two plate members are coated with black paint to thereby form a non-reflective surface to prevent the occurrence of a flare due to undesired reflection light rays from a periphery of the document.
However, in the image reading apparatus disclosed in the Publication of Hei 6-90328, there is a problem of involving an increased number of production processes due to the formation of a black paint layer on the peripheral members.